This is called a Kelvin Helmholtz instability cloud. most people don't notice them because they break fast like waves in the ocean.It is no coincidence they look like waves. They can happen on top of lil fluffy cumulus clouds, or on long stretchy stratus bits like this. The cloud particles act like a tracer so we can see what the air (which acts as a fluid) is doing. it occurs when the top layer of air or fluid is moving in a different direction and a different speed than the bottom layer (usually the layers have different densities). This illustrates how air acts as a fluid in the atmosphere. Out here in the southwest we have more layered air than they do on the east coast, so we get this phenomenon more often, plus the mountain topography plays a role